Method of sub resolution assist feature

ABSTRACT

The present invention discloses a design method of a sub resolution assist feature, which comprises the following steps of: S01: forming a sub resolution assist feature in the mask plate, the upper surface of the sub resolution assist feature is aligned with the upper surface of the mask plate; S02: forming a process pattern on one side, which contains the sub resolution assist feature, of the mask plate, the position of the process pattern is not superposed with the sub resolution assist feature in a vertical direction.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 15/822,230 filed Nov. 27, 2017, which claims the priority benefit ofChina patent application serial No. 201710928912.9, filed Oct. 9, 2017.The entire contents of the above-mentioned patent application are herebyincorporated by reference herein and made a part of the specifications.

FIELD OF THE INVENTION

The present invention relates to a semiconductor fabrication field, moreparticularly to a design method of a sub resolution assist feature.

BACKGROUND OF THE INVENTION

The rapid development of the semiconductor industry mainly benefits fromthe improvement of the microfabrication technique of the microelectronictechnology, while the lithography technique is one of the most criticaltechniques in the chip fabrication. Since the optical lithographytechnique continuously innovates, it breaks through the optical exposurelimit desired by people, which makes it become a mainstream technologyof current exposure.

The lithography system mainly includes four parts of illumination system(light source), mask, projection system and wafer. Diffraction occurswhen light enters on mask, the diffracted lights after entering theprojection system occur interference and image on the wafer, and afterbeing subjected the development and etching processing, a pattern istransferred into the wafer.

In the actual production, it often confronts the case where it isrequired only a small part of pattern with mask sparse, and when thereis only sparse process pattern on the mask plate, it may causedifference between an isolated imaging and a dense imaging during thelithography. Therefore, the Sub Resolution Assist Feature (SRAF) designis introduced at a process node of 90 nm in lithography. The subresolution assist feature is added around a part of sparse pattern,however the sub resolution assist feature only acts as the functions ofblocking and scattering lights in the exposure process to increase thefocusing depth of the lithography pattern and enlarge the process windowof the lithography, they can't form pattern on the wafer. Therefore, theexisting technique adds sub resolution assist lines less than theminimum resolution of the lithography process on the mask plate.

These sub resolution assist lines include various serifs and scatteringbars. The width of the serifs and scattering bars and the distance ofthem from the main feature pattern are more important, and need to beoptimized according to the specific situations, to achieve the profileadjustment on the space image by influencing the phase spectrum with thescattering bars. These scattering bars or serifs, by improving theenergy and phase distribution of various frequency components in thepattern spectrum, can effectively adjust the light intensitydistribution of the space image without forming pattern on the etchingresist, and can act as the functions of improving the linewidthdeviation, strengthening the corner profile and increasing focusingdepth of exposure.

The conventional design method of the SRAF is shown in FIG. 1, andspecifically comprises the following steps of:

(1) depositing a metal layer on a surface of a mask plate;

(2) defining a sub resolution assist feature and a process pattern onthe metal layer, and performing exposure;

(3) etching a portion other than the sub resolution assist feature andprocess pattern on the metal layer, to form the sub resolution assistfeature and the process pattern which are located on the same horizontalplane.

In the procedure that the mask plate fabricated according to the aboveprocess is used as a mask, since the sub resolution assist feature andthe process pattern are located on the same horizontal plane, thefocusing planes of the sub resolution assist feature and the processpattern are also located on the same horizontal plane correspondingly,as shown in FIG. 2. This results in that the sub resolution assistfeature may cause a ghost defect on the silicon wafer during theexposure, thereby influence the exposure quality of the silicon wafer.

In addition, the size of the sub resolution assist feature can't be toosmall either, and if the feature is too small, it can't effectivelyblock the incident light, and may even deteriorate the process window ofthe lithography pattern. While in the procedure of the above processfabrication, if sub resolution assist feature is made become small, thiswill increase the cost of the mask lithography, thereby increase thefabrication cost of the mask plate. Therefore, in the procedure of thesub resolution assist feature design, the size of the sub resolutionassist feature shall be appropriately controlled.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is toprovide a design method of a sub resolution assist feature, in which thesub resolution assist feature is set in a mask plate and a processpattern is set on the mask plate, so as to avoid the sub resolutionassist feature and the process pattern being located on the samefocusing plane, reduce a risk of occurring the ghost, control the sizeof sub resolution assist feature better, and enhance the scatteringeffect.

To achieve the above objects, the present invention adopts the followingtechnical solution: a design method of a sub resolution assist feature,comprising the following steps of:

S01: forming a sub resolution assist feature in the mask plate, theupper surface of the sub resolution assist feature is aligned with theupper surface of the mask plate;

S02: forming a process pattern on one side, which contains the subresolution assist feature, of the mask plate, the position of theprocess pattern is not superposed with the sub resolution assist featurein a vertical direction.

Further, the sub resolution assist feature is added around a sparse partof the process pattern.

Further, the thickness of the sub resolution assist feature is less thanor equal to the thickness of the mask plate.

Further, the step S01 is specifically as follows:

S0101: defining a desired pattern of the sub resolution assist featureon the surface of the mask plate, and performing an exposure to form thedesired pattern;

S0102: transferring the desired pattern into the mask plate after beingetching processing;

S0103: growing a metal layer in the desired pattern to form the subresolution assist feature.

Further, in the step S0102, the desired pattern after the exposure isetched by using a dry etching.

Further, in the step S0103, after growing the metal layer in the desiredpattern, the upper surface of the sub resolution assist feature is madeto be aligned with the surface of the mask plate by an etch-back manner.

Further, the metal layer is chromium.

Further, the step S02 is specifically as follows:

S0201: growing a metal layer on the surface of one side, which containsthe sub resolution assist feature, of the mask plate;

S0202: defining the process pattern on the metal layer, and performingexposure;

S0203: etching the metal layer other than the process pattern to formthe process pattern.

Further, the material of the metal layer is chromium.

The mask for lithography provided in the present invention includes amask plate, a sub resolution assist feature and a process pattern,wherein the sub resolution assist feature is located in the mask plate,the upper surface of the sub resolution assist feature is aligned withthe surface of the mask plate, and the process pattern is located on thesurface of one side, which contains the sub resolution assist feature,of the mask plate, and wherein the position of the process pattern isnot superposed with the sub resolution assist feature in the verticaldirection, and when the mask plate for lithography is used as a mask,the incident light enters from the back of the process pattern and thesub resolution assist feature, and performs exposure on the wafer afterrespectively passing through the sub resolution assist feature and theprocess pattern.

Further, the sub resolution assist feature and the process pattern arenot on the same focusing plane.

The beneficial effects of the present invention are as follows: thepresent invention, by setting the sub resolution assist feature and theprocess pattern on two different planes, wherein the sub resolutionassist feature is located in the mask plate and the process pattern islocated on the surface of one side containing the process assist featureof the mask plate, avoids the sub resolution assist feature and theprocess pattern being located on the same focusing plane, so as toachieve the effect of reducing the occurrence of ghost. In addition, thesub resolution assist feature and the process pattern are individuallyfabricated, which can control the size of the process assist featurebetter, and enhance the scattering effect.

BRIEF DESCRIPTION OF THE DRAWINGS

The optimization method and system for overlay error compensation of thepresent invention will be elucidated by reference to the followingembodiments and the accompanying drawings, in which:

FIG. 1 is a flow chart of fabricating a sub resolution assist featureand a process pattern in the prior art.

FIG. 2 is a schematic diagram in which the sub resolution assist featureand the process pattern have the same focusing plane in the prior art.

FIG. 3 is a flow chart of a design method of a sub resolution assistfeature of the present invention.

FIG. 4 is a schematic diagram in which the sub resolution assist featureand the process pattern have different focusing planes in the presentinvention.

REFERENCE SIGNS

-   -   1 mask plate    -   2 sub resolution assist feature    -   3 process pattern    -   4 silicon wafer

DETAILED DESCRIPTION OF THE EMBODIMENTS

To make the object, the technical solution and the advantage of thepresent invention clearer, the specific embodiments of the presentinvention are described in detail below in combination with drawings.

As shown in FIG. 3, the present invention provides a design method of asub resolution assist feature, comprising the following steps of:

S01: forming a sub resolution assist feature in the mask plate, theupper surface of the sub resolution assist feature is aligned with theupper surface of the mask plate.

Wherein the thickness of the sub resolution assist feature is less thanthe thickness of the mask plate, that is to say, in the presentinvention, the sub resolution assist feature is formed in grooves insidethe mask plate, and the upper surface of the grooves are parallel withthe upper surface of the mask plate.

Wherein, the S01 is specifically as follows:

S0101: defining a desired pattern of the sub resolution assist featureon the surface of the mask plate, and performing an exposure to form thedesired pattern.

The patterned manner of the mask in the prior art can be adopted todefine the position of the sub resolution assist feature on the surfaceof the mask plate, that is, a portion other than the sub resolutionassist feature may be covered by using photoresist, and an exposure isperformed.

When the position and size of the sub resolution assist feature aredefined, it is required to consider the factors such as applicationenvironment of the mask plate, the kinds of incident lights, theposition and density degree of the process pattern and so on.

In addition, the size of the sub resolution assist feature can't be toosmall, and if too small, it will result in that the sub resolutionassist feature can't attain an effect of blocking the incident light,and the process window of the sub resolution assist feature may evendeteriorate.

S0102: transferring the desired pattern into the mask plate after beingetching processing. This can be performed by any method for etching themask plate in the prior art, and in the present embodiment, the dryetching is used to form the groove. The gas selected in the dry etchingis gas for etching the mask plate in the prior art.

S0103: growing a metal layer in the desired pattern to form the subresolution assist feature. Wherein the metal layer can be chromium.

And after the metal layer is grown in the groove of the desired pattern,the metal grown in the groove will exceed the surface of the mask plate.At this point, the unnecessary metal which is higher than the plane ofthe mask plate in the groove is removed by an etch-back manner, so thatthe upper surface of the sub resolution assist feature is aligned withthe surface of the mask plate.

S02: forming a process pattern on one side, which contains the subresolution assist feature, of the mask plate, the position of theprocess pattern is not superposed with the sub resolution assist featurein a vertical direction.

The step S02 is specifically as follows:

S0201: growing a metal layer on the surface of one side, which containsthe sub resolution assist feature, of the mask plate.

In this step, the upper surface of the sub resolution assist feature isaligned with the upper surface of the mask layer. That is, the uppersurface of the mask plate containing the sub resolution assist featureis planar, and the metal layer is grown directly on the surface of oneside, which contains the sub resolution assist feature, of the maskplate. The metal layer may be any metal capable of being used as theprocess pattern, and the metal chromium is selected in the presentembodiment.

S0202: defining the process pattern on the metal layer, and performingexposure.

In the present invention, the patterned process in the prior art may beadopted to define the position of the process pattern on the metallayer. In the present embodiment, a photoresist is used to cover theposition of the process pattern, and an exposure is performed, todetermine the position of the process pattern on the metal layer.

S0203: etching the metal layer other than the process pattern to formthe process pattern.

The metal layer other than the process pattern is etched by using thephotoresist as mask and the etching is stopped at the surface of themask plate. Thus, a new mask plate containing a mask plate, a subresolution assist feature and a process pattern is formed. Moreover, thesub resolution assist feature and the process pattern are located ondifferent planes of the mask plate, thereby avoid the sub resolutionassist feature and the process pattern being located on the samefocusing plane, and reduce the risk of occurring ghost.

The mask plate for lithography provided in the present inventionincludes a mask plate, a sub resolution assist feature and a processpattern, wherein the sub resolution assist feature is located in themask plate, the upper surface of the sub resolution assist feature isaligned with the surface of the mask plate, and the thickness of the subresolution assist feature is less than or equal to the thickness of themask plate. The process pattern is located on the surface of one side,which contains the sub resolution assist feature, of the mask plate, andwherein the position of the process pattern is not superposed with thesub resolution assist feature in the vertical direction, that is, thesub resolution assist feature and the process pattern are alternatedeach other in the vertical direction. When the mask plate forlithography is used as a mask, the incident light enters from the backof the process pattern and the sub resolution assist feature, andperforms exposure on the wafer after respectively passing through thesub resolution assist feature and the process pattern.

From the above, the present invention provides a mask plate 1 forlithography, wherein the sub resolution assist feature 2 is located inthe mask plate 1, and the process pattern 3 is located on the surface ofone side, which contains the sub resolution assist feature 2, in themask plate 1. This avoids the sub resolution assist feature 2 and theprocess pattern 3 being located on the same focusing plane whenperforming a mask lithography on the silicon wafer 4, to achieve theeffects of reducing the occurrence of ghost, as shown in FIG. 4.

In addition, the sub resolution assist feature 2 and the process pattern3 are individually fabricated, which can control the size of the processassist feature 2 better, and enhance the scattering effect.

The above is only the preferred embodiment of the present invention. Theembodiment is not intended to limit the patent protection scope of thepresent invention. Therefore, all the equivalent structural changes madeusing the contents of the specification and drawings of the presentinvention, should be encompassed in the protection scope of the presentinvention in a similar way.

The invention claimed is:
 1. A mask plate for lithography, whereincomprising a mask plate, a sub resolution assist feature and a processpattern, wherein the sub resolution assist feature is located in themask plate, the upper surface of the sub resolution assist feature isaligned with the surface of the mask plate, and the process pattern islocated on the surface of one side, which contains the sub resolutionassist feature, of the mask plate, and wherein the position of theprocess pattern is not superposed with the sub resolution assist featurein the vertical direction, and when the mask plate for lithography isused as a mask, the incident light enters from the back of the processpattern and the sub resolution assist feature, and performs exposure onthe wafer after respectively passing through the sub resolution assistfeature and the process pattern.
 2. The mask plate for lithographyaccording to claim 1, wherein the sub resolution assist feature and theprocess pattern are not on the same focusing plane.